Hydrogen diffusion barrier on steel by means of a pulsed-plasma ion-nitriding process

ABSTRACT

Patente of invention for “Hydrogen Diffusion Barrier on Steel by Means of a Pulsed-Plasmalon-Nitriding Process”. The present invention refers to a pulsed-plasma ion-nitriding process performed with the objective of creating hydrogen diffusion barrier on steel, herein exemplified by using the API 5L X-65 steel; high-strength low-alloy steel. The pulsed-plasma ion-nitriding consisted to drive ions and active species of atomic and molecular nitrogen to the material&#39;s surface by applying a difference of potential between two electrodes, periodically interrupted with a pre-determined frequency, such that the cathode ( 1 ) is the own material or piece to be treated, in a chamber ( 2 ) that was previously vacuum pumped ( 4 ) and then filled up ( 6 ) with the gas nitrogen or a gaseous mixture containing this gas.

TECHNICAL FIELD

The present invention concerns a pulsed-plasma ion- nitriding processwith the aim of creating hydrogen diffusion barriers on steels, beingexemplified here for an API 5L X-65 high strength low alloy steel.

PRECEDING PROCEDURES

Conventionally thermo-chemical processes concerning the diffusion of thenon-metallic element nitrogen into the surfaces of engineeringcomponents are carried out by mass transfer using solid, liquid orgaseous environments with the aim of increasing surface hardness.

The gaseous nitriding is among the conventional processes through whichnitrogen is introduced in the surface of the material by dissociatingammonia onto such surface, at temperatures varying between 495 and 565°C., and the liquid nitriding, using fused cyanate and cyanide salt bathsin temperatures between 500 and 575° C. The advent of a nitridingprocess using plasma to drive nitrogen onto the material's surface,substituting the conventional processes, brought many combinedadvantages such as the increase on hardness, wear, fatigue and corrosionresistances, as well as better magnetic properties. The advantagesincluding the process itself include: better control of the material'smicrostructure, and consequently of the desired material's properties;reduction on the energy consumption up to 50% and of the treatment timefrom 30 to 50%; reduction on the gas consumption; elimination ofenvironmental pollution and of risks of explosion and contamination withtoxics wastes, such as cyanide; the possibility to use lowertemperatures in a wide range varying from room temperature to 400° C.,preferentially in temperatures between 300 and 400° C., thereforedecreasing structural distortions and phase changes.

The ion-nitriding may be obtained by using continuous or pulsed currentwith varied frequencies. Basically, the difference between thecontinuous and the pulsed mode is the interruption of the appliedvoltage, which brings benefits making the pulsed-plasma ion-nitridingprocess to present advantages as compared to the continuous- plasmaprocess, such as the reduction on the amount of ions that reach thesample surface, by converting them into neutral atoms through therecombination with electrons during the interruption of the electricdischarge, therefore increasing the efficiency of the process andreducing the cathodic sputtering of the material's surface.

DETAILED DESCRIPTION OF THE INVENTION

The innovation herein proposed describes a pulsed- plasma ion-nitridingprocess that consists to guide ions and active species of atomic andmolecular nitrogen to the surface of the material, by means of applyinga potential difference between two electrodes, which is periodicallyinterrupted with a pre-determined frequency, being the cathode thematerial itself (or component) to be treated, in a previously evacuatedchamber into which the gas nitrogen or a gaseous mixture containing thisgas is introduced. A potential difference is applied for a certain time,the discharge time t_(d), and interrupted for another period of time,the post-discharge time t_(pd), creating a glow discharge that assuresboth a total coverage of the cathode and sufficient heat to the materialto be nitrided that an external heat source may not be necessary. Thepercentage of the pulse in which the voltage is applied is known asactive time t_(a). During the time in which the potential difference isapplied electrical discharges are produced, generating plasma (ionizedgas). In these conditions working gas, nitrogen, ions are created, whichare driven by the potential difference to the cathode, the piece to betreated.

Surface modifications are created in the material, generating twodistinct layers: the white layer or composed layer, made out of ironnitrides, followed by the diffusion zone that contains nitrogen in solidsolution into the ferrite and iron nitrides. Besides obtaining bettersurface properties, such as the increase on hardness, corrosionresistance and fatigue resistance, the present work proposes thepulsed-plasma ion-nitriding as a process to reduce the hydrogenpermeability through the material. This was exemplified by using the API5L X-65 steel, with the chemical composition depicted in Table 1, as amodel to present the effects of pulsed-plasma ion-nitriding, speciallythose related to hydrogen. The samples were pulsed-plasma ion- nitridedon only one of their sides. TABLE 1 Chemical composition of the API 5LX-65 steel (wheight %). C 0.11 Si 0.29 Nb 0.032 Mn 1.05 Al 0.035 V 0.055S 0.005 Ni 0.15 Ti 0.010 p 0.014 Cu 0.31 Ca 0.00747 Fe - balanceThe first step of the pulsed-plasma ion-nitriding process consisted ofpositioning the sample (1) that is the cathode itself into the nitridingchamber (2), whose internal wall is the anode (3), evacuated by a vacuumpump (4) until the pressure gauge (5) indicated a pressure of, forexample, 30 mTorr (3.99×10⁻⁶ MPa). A gas inlet (6) allowed theintroduction of a gaseous mixture rich in nitrogen, in percentages thatvaried in the range, although the gaseous mixture preferentially usedwas in the range N₂+0%-20% H₂, and a working pressure of, for example, 4Torr (5.33×10⁻⁴ MPa) was chosen. The potential difference (7) wasapplied in such a way that the temperature within the chamber was, forexample, in the range 300 to 400° C., measured by a thermocouple (8).The nitriding times were evaluated by summing the periods of time inwhich the plasma was active, in order to maintain this total time at afixed value. Upon finishing the nitriding, the samples were cooled downin the nitriding chamber in a nitrogen atmosphere. FIG. 1 presents aschematic arrangement of the pulsed-plasma ion-nitriding system used.

Examples of conditions used in the pulsed-plasma ion-nitriding of theAPI 5L X-65 steel:

-   -   Frequency equal to about 100 Hz; active time between 40 and 80%;        nitriding time in the range of 4 to 8 hours; discharge time of        around 4.0 to 8.0 ms; post-discharge time between 2 and 6 ms;        potential difference in the range of 360 to 410 V; and current        density between 3.0 and 5.0 mA.cm⁻²;    -   Frequency equal to about 500 Hz; active time between 50 and 80%;        nitriding time in the range of 3 to 6 hours; discharge time of        around 1.0 to 2.0 ms; post-discharge time between 0.2 and 1.0        ms; potential difference in the range of 350 to 400 V; and        current density between 3.0 and 5.0 mA.cm⁻²;        Experimental Techniques

The double-potentiostatic electrochemical method was the technique usedfor the determination of hydrogen permeability in metallic materials.However, a step was required before the permeation, the potentiodynamicpolarization test, with the aim of defining the cathodic potential orcurrent for hydrogen generation, to be used in the permeation test.

The potentiodynamic polarization test consisted on the application of apotential ramp, varying at a rate of, for example, 600 mV.h⁻, betweenthe work electrode that was the sample to be analyzed and the platinumcounter electrode, displacing it with respect to the open circuitpotential (the approximately constant open circuit potential measuredbetween the work electrode and the saturated calomel referenceelectrode) to the direction of positive potential values, anodic, or tothe direction of negative potential values, cathodic, depending on theanalysis to be made, while the resulting current was monitored. Duringthe test a convenient electrolyte was used, for example, a 0.1 N NaOHsolution that was bubbled with gas nitrogen. The electrochemicalreactions that may take place during the application of the potential inthe range −2V to +2V are, respectively, the reduction reaction, throughwhich the sample is reduced by gaining electrons (cathodic polarization)and the oxidation reaction, through which the sample is oxidized byloosing electrons (anodic polarization).

The hydrogen permeation parameters were determined from electrochemicalhydrogen permeation tests with cathodic charging making use of aprogrammable electrochemical interface that allowed the control ofparameters and data acquisition by means of a microcomputer and a twocompartment electrochemical cell, presenting one side to generatehydrogen and the other for its detection. With such an apparatuscurrents and potentials were measured and applied with resolutions of 1nA and 0.1 mV, respectively. The temperature was thermostaticallycontrolled and measured with silicon transistors, with a resolution of0.01° C., guaranteeing temperature variations during the test smallerthan +/−0.1° C.

For the nitrided samples, the tests were conducted following twodifferent orientations: generating hydrogen on the sample's nitridedface and detecting it on the sample's non-nitrided (substrate) face and,conversely, generating hydrogen on the sample's non-nitrided (substrate)face and detecting it on the sample's nitrided face. Electrochemicalhydrogen tests were also conducted using non-nitrided samples with theobjective of obtaining the substrate's permeation parameters. All testsherein shown with the objective of exemplifying the role of hydrogendiffusion barrier played by the nitrided layer were conducted at thetemperature of 50° C.

Results

Curves of hydrogen permeation parameter versus time were plotted basedon the results obtained from the hydrogen permeation tests. The hydrogenpermeation parameter is equal to the product of the hydrogen flux by thesample thickness for each time during a test. The hydrogen permeationparameters for the pulsed-plasma ion-nitrided samples were obtained intwo different ways: by generating hydrogen on the nitrided face anddetecting it on the substrate face (curve marked P_(ns), on FIGS. 2 and3) and, conversely, by generating hydrogen on the substrate face anddetecting it on the nitrided face (curve marked P_(sn) on FIGS. 2 and3). FIGS. 2 and 3 exemplify two specific pulsed-plasma ion-nitridingconditions: using frequencies of 100 Hz and 500 Hz with active times of60% and 50%, respectively. FIG. 2 presents the hydrogen permeationcurves for the substrate steel, P_(s), and for the pulsed-plasmaion-nitrided steel (P_(ns), P_(sn)) for a frequency of 100 Hz and anactive time equal to 60%. FIG. 3 presents the hydrogen permeation curvesfor the substrate steel, P_(s), and for the pulsed-plasma ion-nitridedsteel (P_(ns), P_(sn)) for a frequency of 500 Hz and an active timeequal to 50%.

Table 2 relates the hydrogen permeation parameters for the as-receivedsubstrate API X-65 steel and for this steel after pulsed-plasmaion-nitriding with frequencies of 100 Hz and 500 Hz with active timesequal to 60% and 50%, respectively. TABLE 2 Hydrogen PermeationParameters for the as-received substrate and for the pulsed-plasmaion-nitriding API X-65 steel using frequencies of 100 Hz and 500Hz.Active Permeability time Difusivity Solubility P_(∞) Material Frequency(%) (m².s⁻¹) (molH.m⁻³) (molH.m⁻¹.s⁻¹) Substrate — — 2.47 × 10⁻¹⁰ 0.81P_(∞s) = 2.10 × 10⁻¹⁰ Steel Pulsed- 100 Hz 60 1.44 × 10⁻¹² — P_(∞ns) =5.20 × 10⁻¹¹ Plasma Ion- P_(∞sn) = 1.17 × 10⁻¹² Nitrided 500 Hz 50 2.16× 10⁻¹² P_(∞ns) = 5.29 × 10⁻¹³ Steel P_(∞sn) = 1.75 × 10⁻¹²P_(∞)= the material's hydrogen permeability that is equal to the productof the hydrogen flux (higher plateau of the hydrogen permeation curve)by the sample thickness. It represents the maximum value the hydrogenpermeation parameter may reach in each case.P_(∞ns) = the hydrogen permeability in the material when hydrogen isgenerated on the nitrided layer and it is detected on the substrateduring the electrochemical hydrogen permeation test.P_(∞sn) = the hydrogen permeability in the material when hydrogen isgenerated on the substrate and it is detect on the nitrided layer duringthe electrochemical hydrogen permeation test.

Analysis of the hydrogen permeation curves and parameters for thesubstrate steel (P_(s)) and for the pulsed- plasma ion-nitrided steel(P_(-ns) and P_(-ns)) showed that the hydrogen permeability through thepulsed-plasma ion-nitrided steel is hundreds of times smaller than thatverified through the substrate steel. Thus, the pulsed plasma ionnitriding consisted of an adequate method to create a diffusion barrierfor hydrogen in steel. The decrease of the hydrogen permeation throughthe material is important to limit its hydrogen contamination and, as aresult, the risk of hydrogen embrittlement. The on-service hydrogencontamination of the steel is facilitated because hydrogen is an elementbearing the smallest atomic diameter, thus being very mobile through thematerial's structure by solid state diffusion. The deleterious effect ofhydrogen modifies the contaminated material's mechanic-metallurgicalproperties, by reducing its ductility and fracture stress.

Such contamination may occur upon different situations involvingreactions that liberate hydrogen on the metal's surface, as well as inhydrogen rich environments, such as those that are subject themechanical components in petrochemical, chemical and nuclear industriesor yet during fabrication and thermo-chemical processing, as well asupon the corrosion of steels.

1- Pulsed-plasma ion-nitriding process characterized by positioning thesample (1) that is the cathode itself, in the interior of a nitridingchamber (2), whose internal wall is the anode (3), wherein vacuum ismade by means of a vacuum pump (4) until the pressure gauge (5) reads apressure of, for example, equal to 30 mTorr (3.99×10⁻⁶ MPa), in whichchamber a gas inlet (6) is used to introduce a nitrogen rich gaseousmixture with composition varying in the range N₂₊₀%-50% H₂, choosing awork pressure of, for example, about 4 Torr (5.33×10⁻⁴ MPa), andapplying a difference of potential (7) that corresponds to a temperatureof up to 400° C. measured by means of a thermocouple (8), such that thenitriding times are calculated from the sum of the periods of time thatthe plasma was active, so as to keep this total time a fixed value, andafter finishing the nitriding treatment the samples are cooled withinthe nitriding chamber under a nitrogen atmosphere. 2- “Pulsed-PlasmaIon-Nitriding Process” in accordance with reinvindication 1,characterized to be a method to obtain a diffusion barrier for hydrogenin steel. 3- “Pulsed-Plasma Ion-Nitriding Process” in accordance withreinvindication 1, characterized to be performed in steel using anextended range of temperatures, from room temperature to 400° C.,preferentially in temperatures between 300 and 400° C. 4- “Pulsed-PlasmaIon-Nitriding Process” in accordance with reinvindication 1, by makinguse of a gaseous mixture preferentially for the example disclosed in therange N₂+0%-20% H₂. 5- “Pulsed-Plasma Ion-Nitriding Process” inaccordance with reinvindication 1, characterized by calculating thenitriding times from the summation of the times in which the plasma wasactive, in order to keep this total time at a fixed value. 6-“Pulsed-Plasma Ion-Nitriding Process” in accordance with reinvindication1, characterized by measuring the hydrogen permeability in thepulsed-plasma ion-nitrided steel hundreds of times smaller than thehydrogen permeability in the substrate steel.